Sowing Seeds in a Magnetic Field
Scientists hope that an unusual experiment slated for launch on the space shuttle
this summer will reveal how plants know up from down.
When gardeners poke a seed into the ground, they never worry in which direction
it lays. Give it enough water and food and care, and sure enough, its root will
grow downward and its stem will sprout upward -- every time! Lay the seed upside-down,
and the root and stem would still find their proper positions.
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Top right: The seeds of flax plants will be sprouted
in orbit to help figure out how plants sense gravity. Image courtesy Flax
Council of Canada.
How do plants do it? We humans know up from down (even with our eyes closed)
because we have a complex organ in our inner ear that senses gravity's pull and
signals the brain. But plants have no such organ. It's a puzzle.
Everyone knows that plants grow toward light, but there must be more to it
than that. Trees in northern forests, for example, grow straight up even though
the Sun is never directly overhead, and the first stem emerging from a buried
seed grows upward through dark soil.
It's clear that gravity must play some role, too. Indeed, scientists know that
the direction of gravity's pull is behind many plant behaviors, such as corn crops
righting themselves after being flattened by a storm. What's unclear is exactly
how plants "feel" gravity and respond to it. What part of a plant senses
the direction of gravity's pull? And how is that pull translated into a chemical
response that alters the plant's growth? 
No one knows the answers. But scientists do know enough to suggest two possibilities.
First, when the fluid contents of plant cells (called the "protoplasm")
are pulled downward by gravity, the pressure exerted on the cell walls might serve
as a signal that helps plants distinguish up from down. Second, plant cells contain
starch grains which, like protoplasm, drift down when gravity is present. Scientists
suspect this might act as a cue to plants, too.
But which is it? A novel experiment slated to fly aboard the space shuttle
in July 2002 (STS-107) might reveal the answer.
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Karl Hasenstein, principal investigator for the BioTube/Magnetic Field Apparatus
experiment, explains: The shuttle will carry a payload of flax seeds to orbit.
Once there, a computer-controlled dose of water will start them growing. Unlike
flax sprouts growing on Earth, these won't feel the usual pull of gravity. The
protoplasm and the starch grains within their cells will float rather than sink.
Plants have been grown in space before. But this experiment will be the first
to subject plants to an "artificial gravity" created by magnets.
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Right: Seen under a microscope, the starch grains in these
plants cells are visible as small dots. Image courtesy NASA.
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The experiment will have a high-gradient magnetic field in the plant growth
chamber. Within the cells of the plants, the protoplasm will be essentially unaffected
by the magnet, but the starch grains will feel the magnetic force. They will sink
to the bottom of the cell as if drawn there by gravity.
Starch grains are not magnetic in the usual sense -- if you held one against
your refrigerator it wouldn't stick. But the grains are "diamagnetic,"
which means they develop a weak magnetic field when other magnets are nearby.
The diamagnet's field will naturally oppose that of the nearby magnet -- hence
the prefix "dia" -- so the starch grains will be repelled. Although
the effect is weak, this diamagnetic response allows researchers to use magnets
to move the starch grains.
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Left: This plant originally sprouted with the pot upright
and was later turned on its side. The new stem growth curved to re-align with
gravity. Image courtesy University of Wisconsin-Madison.
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"By changing only the internal displacement of the starch grains, we can
put one of these two arguments to rest," explains Hasenstein, a professor
at the University of Louisiana at Lafayette. "If the starch grains are the
gravity-sensing trigger, we should see the flax-seed roots curve along the magnetic
gradient. And if the pressure on the cell walls triggers the curvature, we should
see no response."
Infrared cameras will automatically photograph the germinating roots. Regular
cameras can't be used because the chamber will be kept completely dark. The darkness
allows scientists to know that the seeds are responding to the magnetic fields,
not just growing toward a light source.
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Left: The wedge-shaped ferromagnet focuses the Right:
magnetic field lines. Starch grains sedimenting in a narrow tube only 0.3 mm wide
are repelled by the high-gradient field. Image courtesy Oleg Kuznetsov, University
of Louisiana at Lafayette.
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Don't bother trying this experiment at home with ordinary refrigerator magnets.
Only special "high-gradient" magnetic fields will do. Hasenstein's experiment
uses magnets about 50 times more powerful than a typical refrigerator magnet.
The magnets have ferromagnetic wedges attached to them, which focus a strong magnetic
field into a small area. Around that area, the strength of the field tapers off
quickly, creating the "gradient" of field strength that moves the starch
grains.
High-gradient magnetic fields will be used in two chambers of the experiment,
while a third chamber will use a homogeneous magnetic field as a "control."
The lessons learned won't only apply to flax seeds (which were chosen for their
small size and their quick, reliable germination). All normal plants have these
starch grains, so the results of this experiment will add to our basic understanding
of plants in general.
Starch grains or protoplasm? No matter which proves correct, researchers will
have lingering questions. For example: "how does the mechanical trigger (e.g.,
starch grains drifting downward) produce a biochemical response?" BioTube/MFA
won't provide all the answers right away, but it is an important first step --
one that will teach us something fundamental about the leafy-green life all around
us.
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Left: Related experiments conducted on the ground used
a slowly rotating device called a "clinostat," to approximate weightlessness.
Gravity pulls on the sprouting seeds from every angle as the device rotates, so
the net effect is nearly zero. These earlier experiments were informative, but
scientists can't be sure of their conclusions without running the experiment in
the true weightlessness of Earth orbit.
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 Download
an audio version of this information.
Credits & Contacts
Authors: Patrick L. Barry, Dr. Tony Phillips
Responsible NASA official: Ron Koczor Production
Editor: Dr. Tony Phillips
Curator: Bryan Walls
Media Relations: Steve Roy
Links to Valuable Resources
Gravitropism
- discusses plants' ability to align themselves with Earth's gravitational field
High-gradient magnetic
fields - describes the sort of magnetic fields used in Hasenstein's experiment.
(A technical description
is also available.)
Visit Karl
Hasenstein's home page
Leafy Green
Astronauts - Science@NASA article: NASA scientists are learning how to grow
plants in space. Such far-out crops will eventually take their place alongside
people, microbes and machines in self-contained habitats for astronauts.
Teaming
up on Space Plants - Science@NASA article: students, scientists, and astronauts
join forces to learn more about how plants grow in space.
Classroom exercise
- laboratory exercise to teach children about plants' responses to gravity (from
the American Society of Plant Biologists).
Classroom
exercise - demonstration of gravitropism suitable for elementary school students.
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